The present invention relates to sonar systems, and more particularly, to a sonar transducer constructed to provide a pair of beams angularly disposed relative to a primary axis without conventional phasing electronics.
In the past, ship mounted sonar systems have existed in which the ocean bottom is used to reflect the sonar energy back to the ship. By measuring the Doppler shift, the true ship's speed over the bottom can be determined. Such systems, sometimes called Doppler Speed Logs, have been utilized for navigation in docking of large ships for some time. They enable measurement of a ship's velocity with respect to the bottom independent of land masses and relay stations, in wind and in all types of weather.
In general, prior art Doppler Speed Log sonar systems have included sonar transducers to send and receive the acoustic energy. These transducers are hull mounted similar to echo sounder transducers. Preamplifiers are needed in the vicinity of the receiving transducer to amplify the weak return signal. Electronic circuitry is required to process the returned signal so that the frequency shift can be determined and a velocity may be computed. A method of display is also required to convert the electrical signal to a visual indication that can be used by the ship's crew.
In order for the motion of a ship to cause a frequency shift in a sonar transmission, the sonar beam must have a directional vector aligned with the motion of the ship. Relatively small trim changes of the ship can cause large apparent velocity changes. To eliminate this sensitivity to trim, a system called the Janus Configuration is used. According to this configuration, grating lobes are generated in the plus and minus thirty degree directions relative to a vertical primary axis. The grating lobes extend fore and aft relative to the longitudinal axis of the ship.
Prior art Doppler Speed Log sonar systems have typically been of the pulse type since continuous wave systems cease to operate when the depth exceeds a predetermined amount, for example, 200 feet. This is because as the water becomes deeper, and the number of scattering particles such as air bubbles increases, the scattered signal begins to dominate over the signal reflected from the bottom.
In one prior art pulse Doppler Speed Log sonar system, commercially manufactured by the Marquardt Company, a two-axis transducer is utilized. This transducer has two separate sending and receiving faces, each aligned at an angle relative to the primary axis for generating the grating lobes in the plus and minus thirty degree directions. The transducer is mounted in a housing on the bottom of the ship which creates a cavity where air bubbles can collect and seriously degrade the accuracy of the system.
Another prior art Doppler Speed Log sonar system called the Atlas-Dolog 10 has been commercially available from Krupp GMBH of Bremen, Germany. In that system, separate transmitter and receiver transducers are utilized. Each consists of a large number (72) of lead-zircon-titanate crystals. Each of the crystals has a flat, cylindrical shape. The crystals are embedded in a block of synthetic material. Complex electronic circuitry including drivers and phase shifters is utilized to generate the sonar beams.